1. Field of the Invention
The present invention relates generally to fuel rods for a nuclear reactor and, more particularly, is concerned with a method of controlling weld chamber purge and cover gas atmosphere.
2. Description of the Prior Art
A nuclear reactor core is composed of a multiplicity of fuel assemblies with each fuel assembly comprised of a plurality of nuclear fuel rods. A typical nuclear fuel rod is manufactured by loading a plurality of nuclear fuel pellets and a plenum spring into a cladding tube and applying bottom and top end plugs to opposite ends of the cladding tube. Typically, the end plugs are welded to the ends of the tube in an atmosphere of inert gas which provides a suitable shield or cover gas for performance of arc welding. Argon, helium, and a helium-argon mixture are examples of cover gases which have been used heretofore, with helium ordinarily being the preferred cover gas. Conventional welding operations performed in the manufacture of nuclear fuel rods are disclosed in U.S. Pat. Nos. to Boyko et al (3,683,148), Fink et al (3,725,635), Boyko et al (3,842,238 and 4,003,788) and Duncan et al (4,075,454).
Argon gas has been used heretofore in purging a chamber used in girth welding of the bottom end plugs of fuel rods. However, it has not been used for purging the chamber for girth welding of the top end plugs for the reason that after girth welding is completed the argon gas would be drawn into the fuel rod through an opening axially-extending gas inlet hole in the top end plug due to a vacuum condition created in the fuel rod by the girth welding. The presence of argon in the fuel rod will cause problems subsequently because of adverse effects on the thermal conductivity of the gas mixture inside the fuel rod.
Up to the present, the solution applied has been to exclude the use of argon gas in the girth welding of top end plugs. Helium is thus present in the chamber at all times. This presents problems in terms of the high cost of such helium utilization and end plug rejections caused by occasional arc jumping.
Consequently, the need exists for improvement of the control of the gas atmosphere during girth welding of top end plugs of fuel rods.